<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
  body{
    background-color: gray;
  }
  .big{
    width: 400px;
    height: 400px;
    background-color: pink;
  }
  .small{
    width: 100px;
    height: 100px;
    background-color: red;
    border-radius: 50%;
    overflow-wrap: break-word;  /*超出部分换行*/
  }
</style>
<body>
  <div class="big">
    <div class="small">
      ddddddddddddddddddddddddddddddddddddddddddd
    </div>
  </div>
</body>
</html>